Phase-cut AC power controls are used in a wide variety of applications. Phase-cut AC power controls trim an AC voltage waveform to control the application of power to a load circuit. The phase angle at which the AC voltage is trimmed may be referred to as the “conduction angle” or the “firing angle”.
Some phase-cut power controls comprise circuits that include one or more thyristors, such as a TRIAC or a silicon controlled rectifier (SCR). A property of thyristors is that, once biased for conduction (turned on) by a gating pulse, they will remain in conduction for as long as they continue to conduct more than a threshold amount of current, commonly known as the holding current or hypostatic current. When the current in a thyristor drops below the holding current, the thyristor turns off and requires another gate pulse before it can turn on again.
In some applications, thyristor-based phase-cut power controls are used to deliver controlled power to a load. In some such applications, current drawn by the load may vary over time, due to, for instance, variations in the supply voltage and variations in load impedance. In such applications, the load current may at times be less than the holding current required to maintain thyristor conduction. Where insufficient load current causes a thyristor to come out of conduction, it may occur that a load receives less power than it should (e.g., the load may not receive the power from the portion of the AC cycle for which the thyristor came out of conduction).
Some prior art devices adjust the current drawn by a load in order to maintain a sufficient current to keep a thyristor in conduction. In some applications, this leads to additional power dissipation in the load that is not desired. For instance, dissipating additional power in an LED load causes the light produced by the LED load to be brighter. As a result, this manner of maintaining a thyristor in conduction may limit the extent to which an LED load can be dimmed.
Holding current circuits may be used adjunct to the load in order to draw a holding current from a thyristor. Some holding current circuits draw a constant holding current from the thyristor. Holding circuits that draw current constantly may negatively impact energy efficiency. Such impacts are particularly relevant where the intended load typically draws little power, such as, for example, an LED lighting load.